The service in this case is facilitating a trade. Liquidity is important, but I think it was oversold to pull some of the PR heat off of HFT. Especially since a lot of retail investing advice has turned to index funds, liquidity for individual stocks matters less for them -- it also means they don't have to understand as much financial info before investing (just read the index's prospectus every year vs. reading a whole bunch of company financial filings every quarter).